"Know" is a stretch. Plants respond to attack by releasing chemicals (e.g. nettles and grasses), curling or retracting their leaves (e.g. acacia), or by changing their morphology (e.g. holly); but they have no nervous system - let alone a brain - so it's not like you're killing an animal.
Let's assume for a moment that somehow your salad was conscious. That's an even bigger reason not to eat an animal that has to be fed on plants for a long time.
Everybody needs to eat stuff. And if it is about reducing pain and having a better climate impact, you should plants all the way. A cow eats 50 times the amount of plants that it gives back in meat.
I mean if you stretch the definition enough even cells know they are being eaten. they usually respond by "shit this cant be good, I better press this big red button"
There was a Hungarian cult that convinced others that people can survive by eating light. There were some deaths and was quickly shut down, but they exist forever in anorexia-related jokes.
Let's go to the extremes here: let's say I'm a vegan, and love snakes and want my snake to not eat live mouse, do you think I can feed the snake vegan snake food?
This is all hypothetical as I dislike snakes and love bacon.
Man I was giggling about this a ton of times as it passed by due to the recent admin posting update about all that shit on the deranged vegan sub until just now I realized the comment was from Roge Jogan. Fuck that dumbass and everything he does.